Human milk proteins
Proteins are the third largest macronutrient in human milk, and are highly bioavailable. There are over 400 proteins contained in human milk, which serve as sources of amino acids and essential sources of nitrogen for infants. Furthermore, these non-nutritional bioactive proteins and peptides play a key role in improving the bioavailability of other micronutrients, promoting:
- the development of the infant nervous system and organs,
- the microbial colonization of the gastrointestinal tract,
- the establishment and modulation of immune barriers.
Human milk proteins can be classified into three approximate categories ranging from high to low concentration, as follows:
- whey proteins 60–80%
- caseins 20–40%
- milk fat globule membrane (MFGM) proteins 1–4%
Whey protein in Human Milk
The most prominent proteins in whey protein are α-lactalbumin (α-La) and lactoferrin (LF). α-lactalbumin makes up about 22% of the total proteins; this protein is essential for lactose synthesis, improves intestinal health and immune response in infants, and increases the absorption of mineral such as calcium and zinc. The latter acts as a transferrin, promotes iron absorption and exerts antibacterial, antiviral, immunomodulatory, and anti-inflammatory effects.
Osteopontin (OPN) has received much attention in recent years, and is only 2% of the total protein. It is associated with biomineralization, tissue remodeling, and immune and metabolism regulation. Compared to bovine milk, human milk casein’s composition makes it easier for infants to digest.
Casein in Human Milk
Accounting for about 70% of the casein protein, β-casein (β-CN) can facilitate calcium absorption, while κ-casein (κ-CN) possesses antioxidative and antibacterial activities. In contrast to αS2-casein, found in bovine milk, αS1-casein (αS1-CN) is the only alpha casein in human milk, and is involved in TLR4-mediated immune responses.
Milk fat globule membrane (MFGM) proteins in Human milk
Among the MFGM proteins, butyrophilin (BTN), periodic acid Schiff 6/7(PAS 6/7), fatty acid-binding protein (FABP), and xanthine oxidoreductase (XOR) occupy a relatively high proportion of the total. These proteins have antibacterial and antiviral activity and can help build intestinal immune barriers in infants.
The protein composition during the lactation stages
During the lactation stage, breast milk can be divided into:
- colostrum (0~7 d),
- transitional milk (8~14 d),
- early mature milk (15~180 d),
- and late mature milk (180 d and above).
The protein composition and content of colostrum and transitional milk change greatly, while that of mature milk is relatively stable. In addition, the status of the mothers and the infants, such as nutritional status, delivery mode, baby gender, etc., are important factors affecting the composition of human milk proteins.
Excerpt: Maternal BMI During Lactation Is Associated with Major Protein Compositions in Early Mature Milk
